I have some users who upgraded from Exchange 2007 to exchange 2013. For some reason their reminders (well some of them) have duplicated itself. The users are in online mode and I had them to go to OWA. When they logged into Owa THEYconfirmed the same duplicates. I had the user to close outlook and do outlook.exe /cleanreminders and they have the same problem. The users using Outlook 2010 sp2.